Gabriela G. Loots

Active Conservation of Noncoding Sequences Revealed by Three-Way Species Comparisons

Human and mouse genomic sequence comparisons are being increasingly used to search for evolutionarily conserved gene regulatory elements. Large-scale human-mouse DNA comparison studies have discovered numerous conserved noncoding sequences of which only a fraction has been functionally investigated A question therefore remains as to whether most of these noncoding sequences are conserved because of functional constraints or are the result of a lack of divergence time.

Control of the SOST Bone Enhancer by PTH Using MEF2 Transcription Factors

Expression of the osteocyte‐derived bone formation inhibitor sclerostin in adult bone requires a distant enhancer. We show that MEF2 transcription factors control this enhancer and mediate inhibition of sclerostin expression by PTH.

Parathyroid Hormone (PTH)–Induced Bone Gain Is Blunted in SOST Overexpressing and Deficient Mice

Intermittent parathyroid hormone (PTH) treatment is a potent bone anabolic principle that suppresses expression of the bone formation inhibitor Sost. We addressed the relevance of Sost suppression for PTH‐induced bone anabolism in vivo using mice with altered Sost gene dosage. Six‐month‐old Sost overexpressing and 2‐month‐old Sost deficient male mice and their wild‐type littermates were subjected to daily injections of 100 µg/kg PTH(1–34) or vehicle for a 2‐month period. A follow‐up study was performed in Sost deficient mice using 40 and 80 µg/kg PTH(1–34). Animals were sacrificed 4 hours after the final PTH administration and Sost expression in long bone diaphyses was determined by qPCR. Bone changes were analyzed in vivo in the distal femur metaphysis by pQCT and ex vivo in the tibia and lumbar spine by DXA. Detailed ex vivo analyses of the femur were performed by pQCT, µCT, and histomorphometry. Overexpression of Sost resulted in osteopenia and Sost deletion in high bone mass. As shown before, PTH suppressed Sost in wild‐type mice. PTH treatment induced substantial increases in bone mineral density, content, and cortical thickness and in aging wild‐type mice also led to cancellous bone gain owing to amplified bone formation rates. PTH‐induced bone gain was blunted at all doses and skeletal sites in Sost overexpressing and deficient mice owing to attenuated bone formation rates, whereas bone resorption was not different from that in PTH‐treated wild‐type controls. These data suggest that suppression of the bone formation inhibitor Sost by intermittent PTH treatment contributes to PTH bone anabolism.

ECRbase: database of evolutionary conserved regions, promoters, and transcription factor binding sites in vertebrate genomes

Evolutionary conservation of DNA sequences provides a tool for the identification of functional elements in genomes. We have created a database of evolutionary conserved regions (ECRs) in vertebrate genomes, entitled ECRbase, which is constructed from a collection of whole-genome alignments produced by the ECR Browser. ECRbase features a database of syntenic blocks that recapitulate the evolution of rearrangements in vertebrates and a comprehensive collection of promoters in all vertebrate genomes generated using multiple sources of gene annotation. The database also contains a collection of annotated transcription factor binding sites (TFBSs) in evolutionary conserved and promoter elements. ECRbase currently includes human, rhesus macaque, dog, opossum, rat, mouse, chicken, frog, zebrafish and fugu genomes. It is freely accessible at http://ecrbase.dcode.org.

Hypoxia Decreases Sclerostin Expression and Increases Wnt Signaling in Osteoblasts

Mutations in sclerostin function or expression cause sclerosing bone dysplasias, involving decreased antagonism of Wnt/Lrp5 signaling. Conversely, deletion of the VHL tumor suppressor in osteoblasts, which stabilize HIF‐α isoforms and thereby enables HIF‐α/β‐driven gene transcription, increases bone mineral content and cross‐sectional area compared to wild‐type controls. We examined the influence of cellular hypoxia (1% oxygen) upon sclerostin expression and canonical Wnt signaling. Osteoblasts and osteocytes cultured under hypoxia revealed decreased sclerostin transcript and protein, and increased expression and nuclear localization of activated β‐catenin. Similarly, both hypoxia and the hypoxia mimetic DFO increased β‐catenin gene reporter activity. Hypoxia and its mimetics increased expression of the BMP antagonists gremlin and noggin and decreased Smad‐1/5/8 phosphorylation. As a partial explanation for the mechanism of regulation of sclerostin by oxygen, MEF2 reporter assays revealed decreased activity. Modulation of VEGF signaling under normoxia or hypoxia revealed no influence upon Sost transcription. These data suggest that hypoxia inhibits sclerostin expression, through enhanced antagonism of BMP signaling independent of VEGF. J. Cell. Biochem. 110: 457–467, 2010.

Targeted deletion of Sost distal enhancer increases bone formation and bone mass

The Wnt antagonist Sost has emerged as a key regulator of bone homeostasis through the modulation of Lrp4/5/6 Wnt coreceptors. In humans, lack of Sclerostin causes sclerosteosis and van Buchem (VB) disease, two generalized skeletal hyperostosis disorders that result from hyperactive Wnt signaling. Unlike sclerosteosis, VB patients lack SOST coding mutations but carry a homozygous 52 kb noncoding deletion that is essential for the transcriptional activation of SOST in bone. We recently identified a putative bone enhancer, ECR5, in the VB deletion region, and showed that the transcriptional activity of ECR5 is controlled by Mef2C transcription factor in vitro. Here we report that mice lacking ECR5 or Mef2C through Col1-Cre osteoblast/osteocyte-specific ablation result in high bone mass (HBM) due to elevated bone formation rates. We conclude that the absence of the Sost-specific long-range regulatory element ECR5 causes VB disease in rodents, and that Mef2C is the main transcription factor responsible for ECR5-dependent Sost transcriptional activation in the adult skeleton.

Global gene expression analysis of murine limb development.

Detailed information about stage-specific changes in gene expression is crucial for understanding the gene regulatory networks underlying development and the various signal transduction pathways contributing to morphogenesis. Here we describe the global gene expression dynamics during early murine limb development, when cartilage, tendons, muscle, joints, vasculature and nerves are specified and the musculoskeletal system of limbs is established. We used whole-genome microarrays to identify genes with differential expression at 5 stages of limb development (E9.5 to 13.5), during fore- and hind-limb patterning. We found that the onset of limb formation is characterized by an up-regulation of transcription factors, which is followed by a massive activation of genes during E10.5 and E11.5 which levels off at later time points. Among the 3520 genes identified as significantly up-regulated in the limb, we find ∼30% to be novel, dramatically expanding the repertoire of candidate genes likely to function in the limb. Hierarchical and stage-specific clustering identified expression profiles that are likely to correlate with functional programs during limb development and further characterization of these transcripts will provide new insights into specific tissue patterning processes. Here, we provide for the first time a comprehensive analysis of developmentally regulated genes during murine limb development, and provide some novel insights into the expression dynamics governing limb morphogenesis.

Absence of sclerostin adversely affects B-cell survival.

Increased osteoblast activity in sclerostin‐knockout (Sost−/−) mice results in generalized hyperostosis and bones with small bone marrow cavities resulting from hyperactive mineralizing osteoblast populations. Hematopoietic cell fate decisions are dependent on their local microenvironment, which contains osteoblast and stromal cell populations that support both hematopoietic stem cell quiescence and facilitate B‐cell development. In this study, we investigated whether high bone mass environments affect B‐cell development via the utilization of Sost−/− mice, a model of sclerosteosis. We found the bone marrow of Sost−/− mice to be specifically depleted of B cells because of elevated apoptosis at all B‐cell developmental stages. In contrast, B‐cell function in the spleen was normal. Sost expression analysis confirmed that Sost is primarily expressed in osteocytes and is not expressed in any hematopoietic lineage, which indicated that the B‐cell defects in Sost−/− mice are non‐cell autonomous, and this was confirmed by transplantation of wild‐type (WT) bone marrow into lethally irradiated Sost−/− recipients. WT→Sost−/− chimeras displayed a reduction in B cells, whereas reciprocal Sost−/−→WT chimeras did not, supporting the idea that the Sost−/− bone environment cannot fully support normal B‐cell development. Expression of the pre‐B‐cell growth stimulating factor, Cxcl12, was significantly lower in bone marrow stromal cells of Sost−/− mice, whereas the Wnt target genes Lef‐1 and Ccnd1 remained unchanged in B cells. Taken together, these results demonstrate a novel role for Sost in the regulation of bone marrow environments that support B cells.

Prostaglandin E2 Signals Through PTGER2 to Regulate Sclerostin Expression

The Wnt signaling pathway is a robust regulator of skeletal homeostasis. Gain-of-function mutations promote high bone mass, whereas loss of Lrp5 or Lrp6 co-receptors decrease bone mass. Similarly, mutations in antagonists of Wnt signaling influence skeletal integrity, in an inverse relation to Lrp receptor mutations. Loss of the Wnt antagonist Sclerostin (Sost) produces the generalized skeletal hyperostotic condition of sclerosteosis, which is characterized by increased bone mass and density due to hyperactive osteoblast function. Here we demonstrate that prostaglandin E2 (PGE2), a paracrine factor with pleiotropic effects on osteoblasts and osteoclasts, decreases Sclerostin expression in osteoblastic UMR106.01 cells. Decreased Sost expression correlates with increased expression of Wnt/TCF target genes Axin2 and Tcf3. We also show that the suppressive effect of PGE2 is mediated through a cyclic AMP/PKA pathway. Furthermore, selective agonists for the PGE2 receptor EP2 mimic the effect of PGE2 upon Sost, and siRNA reduction in Ptger2 prevents PGE2-induced Sost repression. These results indicate a functional relationship between prostaglandins and the Wnt/β-catenin signaling pathway in bone.

Long-term administration of AMD3100, an antagonist of SDF-1/CXCR4 signaling, alters fracture repair.

Fracture healing involves rapid stem and progenitor cell migration, homing, and differentiation. SDF‐1 (CXCL12) is considered a master regulator of CXCR4‐positive stem and progenitor cell trafficking to sites of ischemic (hypoxic) injury and regulates their subsequent differentiation into mature reparative cells. In this study, we investigated the role of SDF‐1/CXCR4 signaling in fracture healing where vascular disruption results in hypoxia and SDF‐1 expression. Mice were injected with AMD3100, a CXCR4 antagonist, or vehicle twice daily until euthanasia with the intent to impair stem cell homing to the fracture site and/or their differentiation. Fracture healing was evaluated using micro‐computed tomography, histology, quantitative PCR, and mechanical testing. AMD3100 administration resulted in a significantly reduced hyaline cartilage volume (day 14), callus volume (day 42) and mineralized bone volume (day 42) and reduced expression of genes associated with endochondral ossification including collagen Type 1 alpha 1, collagen Type 2 alpha 1, vascular endothelial growth factor, Annexin A5, nitric oxide synthase 2, and mechanistic target of rapamycin. Our data suggest that the SDF‐1/CXCR4 signaling plays a central role in bone healing possibly by regulating the recruitment and/or differentiation of stem and progenitor cells.